The present invention relates to a plasma ion source mass spectrometer for trace elements for realizing a quantitative method for trace elements in fields such as material science, etc., and in particular to means for reducing interference of plasma gas ions with isobaric elements to improve the quantification.
A prior art plasma ion source mass spectrometer for trace elements using high frequency plasma is discussed in Analyst, Vol. 108 (Feb. 1983), pp. 159-165. FIG. 2 shows the outline of this prior art device, in which reference numeral 10 is a high frequency oscillator; 20 is a load coil; 30 is a discharge tube; 40 is plasma gas; 50 is auxiliary gas; 60 is a sample; 70 is plasma; 180 is a sampling cone; 190 is a skimmer; 195 is an ion extraction electrode; 200 is an ion beam; 210 is a photon stopper; 220 is an ion lens system; 140 is a slit; 160 is a mass analyzer (quadrupole type); and 170 is an ion detector (channeltron, etc.).
On the other hand, a prior art device using microwave plasma is discussed in Spectrochimica Acta. Vol. 42B, No. 5 (1987), pp. 705-712. The outline of the construction is identical to that indicated in FIG. 2 except for the difference in the plasma generating section.
By the prior art techniques described above, apart from the plasma gas and the auxiliary gas, argon (Ar: mass number 40) gas is used as a carrier gas for the sample. For this reason a number of molecule peaks due to Ar are formed. Therefore there is a problem that for K (mass, m/z=39), Ca(40), Fe(56), etc., which are isobaric elements, the quantification is worsened because of interference with the molecule peaks of Ar, etc. Although it was studied to use an analyzer having a high resolving power as the mass spectrometer in order to reduce this interference, there were problems that the precision was not remarkably improved because of the strong interference, that the analyzer was expensive, etc. Further, although it is studied also to use He in lieu of Ar, because the consumption of He is large, it is expensive and thus it is not practical.